Advanced

import { describe, bench } from "vitest";
// => bench: benchmark function (similar to test)
// => Run with: npx vitest bench
 
describe("array operations benchmark", () => {
  const data = Array.from({ length: 10000 }, (_, i) => i);
  // => Creates array of 10,000 numbers [0, 1, ..., 9999]
  // => Defined outside bench to exclude setup from measurement
 
  bench("Array.map", () => {
    data.map((x) => x * 2);
    // => Measures map operation throughput
    // => Vitest runs this many times and averages
  });
 
  bench("for loop", () => {
    const result = new Array(data.length);
    // => Pre-allocate array
    for (let i = 0; i < data.length; i++) {
      result[i] = data[i] * 2;
      // => Manual loop with index access
    }
  });
 
  bench("Array.reduce", () => {
    data.reduce((acc: number[], x) => {
      acc.push(x * 2);
      return acc;
      // => Build array via reduce
    }, []);
  });
});
 
// Benchmark with options
describe("string concatenation benchmark", () => {
  bench(
    "template literals",
    () => {
      const name = "World";
      const _result = `Hello, ${name}!`;
      // => Template literal interpolation
    },
    { iterations: 1000, warmupIterations: 100 },
    // => iterations: number of measured runs
    // => warmupIterations: runs before measurement starts
  );
 
  bench("string concatenation", () => {
    const name = "World";
    const _result = "Hello, " + name + "!";
    // => Classic string concatenation
  });
});

import { describe, bench } from "vitest";
 
describe("object creation patterns", () => {
  // Pattern 1: Object literal
  bench("object literal", () => {
    const obj = { name: "Alice", age: 30, active: true };
    // => Direct object creation
    // => V8 optimizes with hidden classes
  });
 
  // Pattern 2: Object.create
  bench("Object.create", () => {
    const obj = Object.create(null);
    obj.name = "Alice";
    obj.age = 30;
    obj.active = true;
    // => Prototype-free object
    // => No inherited properties
  });
 
  // Pattern 3: Class instantiation
  bench("class instantiation", () => {
    class User {
      constructor(
        public name: string,
        public age: number,
        public active: boolean,
      ) {}
    }
    const _obj = new User("Alice", 30, true);
    // => Class-based creation
    // => Includes prototype chain
  });
 
  // Pattern 4: Map
  bench("Map creation", () => {
    const map = new Map<string, unknown>();
    map.set("name", "Alice");
    map.set("age", 30);
    map.set("active", true);
    // => Map-based key-value store
    // => Different memory layout than objects
  });
});

import { describe, bench, expect } from "vitest";
 
// Benchmark with assertions (performance SLAs)
describe("performance SLA benchmarks", () => {
  bench("sorting 1000 elements", () => {
    const arr = Array.from({ length: 1000 }, () => Math.random());
    // => Generate random array each iteration
    arr.sort((a, b) => a - b);
    // => Sort ascending
  });
 
  bench("filtering large array", () => {
    const arr = Array.from({ length: 10000 }, (_, i) => i);
    // => Array of 10,000 numbers
    const _result = arr.filter((x) => x % 2 === 0);
    // => Keep even numbers (5,000 results)
  });
 
  bench(
    "JSON parse/stringify round-trip",
    () => {
      const data = { users: Array.from({ length: 100 }, (_, i) => ({ id: i, name: `User ${i}` })) };
      // => Complex nested object
      const _result = JSON.parse(JSON.stringify(data));
      // => Serialize and deserialize
    },
    { time: 1000 },
    // => time: minimum benchmark duration in ms
    // => Ensures enough iterations for statistical significance
  );
});
 
// To save and compare results:
// npx vitest bench --reporter=json --outputFile=bench-results.json
// => Saves results to JSON file for CI comparison
 
// vitest.config.ts benchmark configuration:
// test: {
//   benchmark: {
//     include: ['**/*.bench.ts'],
//     reporters: ['default', 'json'],
//     outputFile: './bench-results.json',
//   },
// }
 
import { test } from "vitest";
 
test("benchmark configuration concept", () => {
  const benchConfig = {
    include: ["**/*.bench.ts"],
    // => File pattern for benchmark files
    reporters: ["default", "json"],
    // => Terminal output and JSON file
    outputFile: "./bench-results.json",
    // => JSON output path for CI
  };
 
  expect(benchConfig.include[0]).toContain(".bench.ts");
  // => Passes: benchmark file pattern set
  expect(benchConfig.reporters).toContain("json");
  // => Passes: JSON reporter enabled
});

// vitest.config.ts - Browser mode configuration
// import { defineConfig } from 'vitest/config';
//
// export default defineConfig({
//   test: {
//     browser: {
//       enabled: true,
//       // => Run tests in real browser
//       provider: 'playwright',
//       // => Use Playwright for browser automation
//       name: 'chromium',
//       // => Browser to use: chromium, firefox, webkit
//       headless: true,
//       // => Run without visible UI (CI-friendly)
//     },
//   },
// });
 
import { test, expect } from "vitest";
 
// Browser mode provides real browser APIs
test("browser mode concept", () => {
  // In browser mode, these are REAL browser APIs:
  // - window.fetch (real fetch, not polyfill)
  // - document.createElement (real DOM, not simulation)
  // - CSS computed styles (real rendering engine)
  // - Web Workers (real threading)
  // - Canvas API (real drawing context)
 
  const browserFeatures = {
    realFetch: true,
    // => fetch() uses real HTTP stack
    realDOM: true,
    // => DOM operations use real rendering engine
    realCSS: true,
    // => getComputedStyle returns real values
    realWorkers: true,
    // => Web Workers run in separate threads
  };
 
  expect(browserFeatures.realFetch).toBe(true);
  // => Browser mode provides real fetch
  expect(browserFeatures.realDOM).toBe(true);
  // => Real DOM instead of happy-dom/jsdom
});
 
test("browser mode vs DOM simulation differences", () => {
  // Behaviors that differ in browser mode vs jsdom:
  const differences = [
    { feature: "CSS transitions", jsdom: false, browser: true },
    // => jsdom doesn't simulate CSS transitions
    { feature: "IntersectionObserver", jsdom: false, browser: true },
    // => jsdom has limited IntersectionObserver
    { feature: "Canvas rendering", jsdom: false, browser: true },
    // => jsdom has stub Canvas implementation
    { feature: "Web Animations", jsdom: false, browser: true },
    // => jsdom doesn't support Web Animations API
  ];
 
  const browserOnlyFeatures = differences.filter((d) => d.browser && !d.jsdom);
  // => Features only available in browser mode
 
  expect(browserOnlyFeatures).toHaveLength(4);
  // => Four features require real browser
});

import { test, expect, vi } from "vitest";
 
// In browser mode, component tests run in real Chromium/Firefox/WebKit
// This provides:
// - Real CSS rendering (computed styles, animations)
// - Real event propagation (bubble, capture)
// - Real layout calculations (offsetWidth, getBoundingClientRect)
 
test("component rendering in browser mode", () => {
  // In browser mode, these measurements are real:
  const element = {
    offsetWidth: 200,
    // => Real pixel width from rendering engine
    offsetHeight: 50,
    // => Real pixel height from rendering engine
    getBoundingClientRect: () => ({
      top: 100,
      left: 50,
      width: 200,
      height: 50,
    }),
    // => Real position from layout engine
  };
 
  expect(element.offsetWidth).toBe(200);
  // => In browser mode: real rendered width
  // => In jsdom: returns 0 (no rendering engine)
 
  const rect = element.getBoundingClientRect();
  expect(rect.width).toBe(200);
  // => Real bounding rectangle from browser
});
 
test("browser mode enables visual assertions", () => {
  // Browser mode supports screenshot comparison:
  // await expect(page.getByTestId('chart')).toHaveScreenshot();
  // => Captures actual rendered pixels
  // => Compares against baseline image
  // => Fails if visual appearance changes
 
  const visualTestingCapabilities = {
    screenshots: true,
    // => Capture element/page screenshots
    pixelDiff: true,
    // => Compare screenshots pixel-by-pixel
    thresholds: { maxDiffPercentage: 0.1 },
    // => Allow 0.1% pixel difference
  };
 
  expect(visualTestingCapabilities.screenshots).toBe(true);
  // => Browser mode enables screenshot testing
  expect(visualTestingCapabilities.thresholds.maxDiffPercentage).toBe(0.1);
  // => Configurable diff threshold
});

// CLI commands for sharding:
// npx vitest run --shard=1/3  => Run first third of tests
// npx vitest run --shard=2/3  => Run second third
// npx vitest run --shard=3/3  => Run final third
 
// GitHub Actions matrix example:
// strategy:
//   matrix:
//     shard: [1/3, 2/3, 3/3]
// steps:
//   - run: npx vitest run --shard=${{ matrix.shard }}
 
import { test, expect } from "vitest";
 
test("sharding distributes tests evenly", () => {
  const totalTests = 300;
  const shardCount = 3;
  // => Three CI runners
 
  const testsPerShard = Math.ceil(totalTests / shardCount);
  // => testsPerShard is 100
 
  expect(testsPerShard).toBe(100);
  // => Passes: 300 / 3 = 100 per shard
 
  // Time savings:
  // Sequential: 300 tests * 0.1s = 30s
  // 3 shards: 100 tests * 0.1s = 10s (3x faster)
  const sequentialTime = totalTests * 0.1;
  const shardedTime = testsPerShard * 0.1;
  const speedup = sequentialTime / shardedTime;
 
  expect(speedup).toBe(3);
  // => Passes: 3x speedup with 3 shards
});
 
test("shard format validation", () => {
  // Shard format: current/total
  const shardSpec = "2/5";
  // => Run shard 2 of 5 total
  const [current, total] = shardSpec.split("/").map(Number);
  // => current is 2, total is 5
 
  expect(current).toBe(2);
  // => Passes: shard index
  expect(total).toBe(5);
  // => Passes: total shard count
  expect(current).toBeLessThanOrEqual(total);
  // => Passes: valid shard index
});

// vitest.config.ts - Pool configuration
// test: {
//   pool: 'threads',
//   // => 'threads': Worker threads (default, fast, shared memory)
//   // => 'forks': Child processes (full isolation, slower)
//   // => 'vmThreads': VM modules in threads (ESM isolation)
//
//   poolOptions: {
//     threads: {
//       maxThreads: 4,
//       minThreads: 1,
//       // => Thread pool size (default: CPU cores - 1)
//     },
//     forks: {
//       maxForks: 4,
//       minForks: 1,
//       // => Process pool size
//     },
//   },
// }
 
import { describe, it, expect } from "vitest";
 
describe("pool configuration concepts", () => {
  it("threads pool - fastest, shared memory", () => {
    // Worker threads share memory with main thread
    // Fast communication via SharedArrayBuffer
    // Potential for shared state leaks between tests
    const threadConfig = {
      pool: "threads",
      isolation: "shared-memory",
      speed: "fastest",
      risk: "state-leaks-possible",
    };
 
    expect(threadConfig.pool).toBe("threads");
    // => Default pool type
    expect(threadConfig.speed).toBe("fastest");
    // => Lowest overhead
  });
 
  it("forks pool - full isolation, slower", () => {
    // Child processes have separate memory
    // Complete isolation between tests
    // Higher overhead for process creation
    const forksConfig = {
      pool: "forks",
      isolation: "full-process",
      speed: "slower",
      risk: "none",
    };
 
    expect(forksConfig.pool).toBe("forks");
    // => Fork-based pool
    expect(forksConfig.isolation).toBe("full-process");
    // => Complete memory isolation
  });
 
  it("vmThreads pool - ESM isolation in threads", () => {
    // VM modules provide module-level isolation
    // Each test gets its own module context
    // Good for ESM-heavy codebases
    const vmConfig = {
      pool: "vmThreads",
      isolation: "module-level",
      speed: "moderate",
      esm: "native",
    };
 
    expect(vmConfig.pool).toBe("vmThreads");
    // => VM thread pool
    expect(vmConfig.esm).toBe("native");
    // => Native ESM support
  });
});

// vitest.config.ts isolation settings
// test: {
//   isolate: true,
//   // => true: each test file runs in isolated worker
//   // => false: test files share workers (faster, less isolated)
//
//   sequence: {
//     shuffle: false,
//     // => false: deterministic order
//     // => true: random order (catches hidden dependencies)
//     seed: 12345,
//     // => Fixed seed for reproducible random order
//   },
//
//   fileParallelism: true,
//   // => true: test files run in parallel
//   // => false: sequential file execution
// }
 
import { describe, it, expect } from "vitest";
 
describe("test isolation strategies", () => {
  it("shuffle detects hidden dependencies", () => {
    // When tests depend on execution order:
    // Test A sets global.user = "Alice"
    // Test B reads global.user (expects "Alice")
    // With shuffle: B may run before A -> fails
    // This reveals hidden test coupling
 
    const tests = ["A: set user", "B: read user", "C: independent"];
    // => Tests with hidden dependency
 
    // Shuffled order might be: [C, B, A]
    // B fails because A hasn't run yet
    const shuffled = [...tests].reverse();
    // => Simulated shuffle (reversed)
 
    expect(shuffled[0]).not.toBe(tests[0]);
    // => Passes: order changed
  });
 
  it("sequential mode for debugging", () => {
    // fileParallelism: false + isolate: false
    // => Slowest but most predictable
    // => All tests run sequentially in one worker
    // => Use for debugging test interaction issues
 
    const debugConfig = {
      isolate: false,
      fileParallelism: false,
      sequence: { shuffle: false },
    };
 
    expect(debugConfig.isolate).toBe(false);
    // => Shared worker (tests see each other's side effects)
    expect(debugConfig.fileParallelism).toBe(false);
    // => One file at a time
  });
});

import { test, expect } from "vitest";
 
// Custom serializer example
// In vitest.config.ts:
// test: {
//   snapshotSerializers: ['./custom-serializer.ts'],
// }
 
// Serializer interface:
// {
//   serialize(val, config, indent, depth, refs, printer),
//   test(val): boolean,
// }
 
// Self-contained serializer demonstration
const dateSerializer = {
  test: (val: unknown) => val instanceof Date,
  // => Only applies to Date objects
  serialize: (val: Date) => `[Date: ${val.toISOString()}]`,
  // => Converts Date to readable ISO string
};
 
test("snapshot serializer concept", () => {
  const date = new Date("2026-04-05T10:30:00Z");
  // => Date object for snapshot
 
  // Without custom serializer:
  // expect(date).toMatchInlineSnapshot('2026-04-05T10:30:00.000Z');
  // => Default serialization may vary by timezone
 
  // With custom serializer:
  const serialized = dateSerializer.serialize(date);
  // => "[Date: 2026-04-05T10:30:00.000Z]"
 
  expect(serialized).toBe("[Date: 2026-04-05T10:30:00.000Z]");
  // => Passes: stable, readable representation
  expect(dateSerializer.test(date)).toBe(true);
  // => Passes: serializer applies to Date objects
  expect(dateSerializer.test("not a date")).toBe(false);
  // => Passes: serializer skips non-Date values
});
 
test("practical snapshot with serializer", () => {
  // Serializers make snapshots readable for complex types
  const event = {
    type: "click",
    timestamp: new Date("2026-04-05T10:30:00Z"),
    target: { id: "button-1", tagName: "BUTTON" },
  };
 
  // With date serializer, snapshot would show:
  // { type: "click", timestamp: [Date: 2026-04-05T10:30:00.000Z], ... }
  // Instead of raw Date object representation
 
  expect(event.type).toBe("click");
  // => Passes: event type is click
  expect(event.timestamp).toBeInstanceOf(Date);
  // => Passes: timestamp is Date object
});

import { test, expect } from "vitest";
 
test("snapshot with property matchers", () => {
  const user = {
    id: Math.random().toString(36).slice(2),
    // => Random ID (different every run)
    name: "Alice",
    // => Static name
    createdAt: new Date().toISOString(),
    // => Current timestamp (different every run)
    role: "admin",
    // => Static role
  };
 
  expect(user).toMatchSnapshot({
    id: expect.any(String),
    // => Match any string for ID (dynamic value)
    createdAt: expect.any(String),
    // => Match any string for timestamp
    // Properties not listed use exact match:
    // name must be "Alice", role must be "admin"
  });
  // => First run: creates snapshot with matchers
  // => Subsequent runs: verifies structure + static values
  // => Dynamic values (id, createdAt) pass if type matches
});
 
test("inline snapshot with matchers", () => {
  const apiResponse = {
    status: 200,
    data: {
      users: [{ id: 1, name: "Alice", lastLogin: new Date().toISOString() }],
    },
    requestId: `req-${Date.now()}`,
    // => Dynamic request ID
  };
 
  expect(apiResponse).toMatchSnapshot({
    requestId: expect.stringMatching(/^req-\d+$/),
    // => Match request ID format
    data: {
      users: [
        {
          lastLogin: expect.any(String),
          // => Any string for last login time
        },
      ],
    },
  });
  // => Validates structure with flexible dynamic values
});

// Debug commands:
// npx vitest --inspect-brk
// => Starts Vitest and pauses at first line
// => Attach debugger (VS Code, Chrome DevTools)
 
// npx vitest --inspect
// => Starts with inspector active but doesn't pause
// => Set breakpoints in IDE, they'll trigger
 
// VS Code launch.json configuration:
// {
//   "type": "node",
//   "request": "launch",
//   "name": "Debug Vitest",
//   "program": "${workspaceRoot}/node_modules/vitest/vitest.mjs",
//   "args": ["run", "--inspect-brk", "--pool=forks"],
//   "console": "integratedTerminal"
// }
 
import { test, expect } from "vitest";
 
test("debuggable test", () => {
  const data = [1, 2, 3, 4, 5];
  // => Set breakpoint on this line
 
  const filtered = data.filter((x) => x > 2);
  // => Step through filter operation
  // => Inspect 'filtered' in debugger: [3, 4, 5]
 
  const mapped = filtered.map((x) => x * 10);
  // => Step through map operation
  // => Inspect 'mapped' in debugger: [30, 40, 50]
 
  expect(mapped).toEqual([30, 40, 50]);
  // => Verify expected output
});
 
test("debugging async operations", async () => {
  // debugger; // Uncomment to break here in debugger
  // => The debugger statement works in Node inspector
  // => Execution pauses when debugger is attached
 
  const result = await Promise.resolve("debug-me");
  // => Set breakpoint, inspect resolved value
  // => Debugger shows promise resolution flow
 
  expect(result).toBe("debug-me");
  // => Step past to see assertion result
});

// Watch mode commands:
// npx vitest              => Start watch mode (default)
// npx vitest --changed    => Only run tests for changed files
 
// Watch mode keyboard shortcuts (while running):
// p => filter by filename
// t => filter by test name
// a => run all tests
// f => run only failed tests
// q => quit
// h => show help
 
import { describe, it, expect } from "vitest";
 
describe("watch mode patterns", () => {
  it("runs on file save", () => {
    // When you save a source file:
    // 1. Vite detects the change (HMR)
    // 2. Vitest finds affected test files
    // 3. Only affected tests re-run
    // 4. Results appear in ~100ms
 
    const watchBehavior = {
      trigger: "file-save",
      // => Watch mode detects file system changes
      scope: "affected-only",
      // => Only re-runs affected test files
      speed: "near-instant",
      // => Vite HMR provides sub-second feedback
    };
 
    expect(watchBehavior.scope).toBe("affected-only");
    // => Smart re-run: only affected tests
  });
 
  it("filters by filename pattern", () => {
    // Press 'p' in watch mode, type pattern:
    // > math
    // Runs only test files matching "math"
    // Useful for focusing on one feature
 
    const filter = { type: "filename", pattern: "math" };
    expect(filter.type).toBe("filename");
    // => Filter by file path
  });
 
  it("runs only failed tests", () => {
    // Press 'f' to re-run only failed tests
    // Useful for fix-verify cycle:
    // 1. See failure
    // 2. Fix code
    // 3. Press 'f' to verify fix
    // 4. Press 'a' to run all
 
    const workflow = ["see-failure", "fix-code", "verify-fix", "run-all"];
    expect(workflow).toHaveLength(4);
    // => Four-step debug cycle
  });
});

// Launch Vitest UI:
// npx vitest --ui
// => Opens browser at http://localhost:51204/__vitest__/
// => Shows visual dashboard with test tree
 
// npx vitest --ui --coverage
// => UI with coverage overlay on source files
 
import { test, expect } from "vitest";
 
test("Vitest UI features", () => {
  const uiFeatures = {
    testTree: true,
    // => Expandable test suite tree view
    sourceView: true,
    // => View test source code inline
    filterBar: true,
    // => Filter tests by name, status, file
    coverageOverlay: true,
    // => Highlight covered/uncovered lines
    moduleGraph: true,
    // => Visualize module import graph
    consoleOutput: true,
    // => View console.log output per test
    diffView: true,
    // => Side-by-side diff for failed assertions
  };
 
  expect(Object.values(uiFeatures).every(Boolean)).toBe(true);
  // => Passes: all features available
  expect(Object.keys(uiFeatures)).toHaveLength(7);
  // => Seven major UI features
});
 
test("Vitest UI module graph", () => {
  // The module graph shows:
  // - Which modules each test imports
  // - Which tests are affected by each module
  // - Import chain from test to source
  // Useful for understanding test scope and coverage gaps
 
  const moduleGraph = {
    testFile: "math.test.ts",
    imports: ["math.ts", "utils.ts"],
    // => Test imports these modules
    affectedBy: ["math.ts", "utils.ts", "constants.ts"],
    // => Changes to these files trigger this test
  };
 
  expect(moduleGraph.imports).toContain("math.ts");
  // => Test imports math module
  expect(moduleGraph.affectedBy).toContain("constants.ts");
  // => Transitive dependency affects test
});

import { test, expect, vi } from "vitest";
 
// Error boundary pattern
function safeJsonParse<T>(json: string, fallback: T): T {
  try {
    return JSON.parse(json) as T;
    // => Attempts to parse JSON
  } catch {
    return fallback;
    // => Returns fallback on parse error
  }
}
 
test("handles valid JSON", () => {
  const result = safeJsonParse('{"name":"Alice"}', { name: "default" });
  // => Parses valid JSON
  expect(result).toEqual({ name: "Alice" });
  // => Passes: parsed correctly
});
 
test("handles invalid JSON with fallback", () => {
  const result = safeJsonParse("not-json", { name: "default" });
  // => Parse fails, returns fallback
  expect(result).toEqual({ name: "default" });
  // => Passes: fallback returned
});
 
test("handles empty string", () => {
  const result = safeJsonParse("", []);
  // => Empty string is invalid JSON
  expect(result).toEqual([]);
  // => Passes: fallback returned
});
 
// Boundary condition testing
function paginate<T>(items: T[], page: number, pageSize: number): T[] {
  // => Paginate array with bounds checking
  if (page < 1) page = 1;
  // => Clamp negative/zero pages to 1
  if (pageSize < 1) pageSize = 10;
  // => Default page size for invalid input
  const start = (page - 1) * pageSize;
  // => Calculate start index
  return items.slice(start, start + pageSize);
  // => Return page of items
}
 
test("pagination boundary conditions", () => {
  const items = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
 
  expect(paginate(items, 1, 3)).toEqual([1, 2, 3]);
  // => Page 1, size 3: first three items
 
  expect(paginate(items, 4, 3)).toEqual([10]);
  // => Page 4: only one item left
 
  expect(paginate(items, 5, 3)).toEqual([]);
  // => Page 5: beyond data range, empty array
 
  expect(paginate(items, 0, 3)).toEqual([1, 2, 3]);
  // => Page 0 clamped to page 1
 
  expect(paginate(items, -1, 3)).toEqual([1, 2, 3]);
  // => Negative page clamped to page 1
 
  expect(paginate(items, 1, 0)).toEqual(items);
  // => Invalid page size defaults to 10
});

// .github/workflows/test.yml
// name: Tests
// on: [push, pull_request]
// jobs:
//   test:
//     runs-on: ubuntu-latest
//     strategy:
//       matrix:
//         shard: [1/4, 2/4, 3/4, 4/4]
//     steps:
//       - uses: actions/checkout@v4
//       - uses: actions/setup-node@v4
//         with:
//           node-version: 22
//           cache: 'npm'
//       - run: npm ci
//       - run: npx vitest run --shard=${{ matrix.shard }} --reporter=junit --outputFile=test-results.xml
//       - uses: dorny/test-reporter@v1
//         if: always()
//         with:
//           name: Vitest Results (Shard ${{ matrix.shard }})
//           path: test-results.xml
//           reporter: java-junit
 
import { test, expect } from "vitest";
 
test("CI configuration concepts", () => {
  const ciConfig = {
    shards: 4,
    // => Split tests across 4 parallel runners
    reporter: "junit",
    // => JUnit XML for CI test result display
    caching: "npm-ci",
    // => Cache node_modules between runs
    failFast: false,
    // => Run all shards even if one fails
  };
 
  expect(ciConfig.shards).toBe(4);
  // => Four parallel test runners
  expect(ciConfig.reporter).toBe("junit");
  // => JUnit format for CI integration
});
 
test("CI environment detection", () => {
  // Vitest detects CI environments automatically
  // process.env.CI is set by most CI providers
  const isCI = process.env.CI === "true";
  // => true in GitHub Actions, GitLab CI, etc.
 
  // CI-specific behavior:
  // - No watch mode (always single run)
  // - No color output (unless terminal supports it)
  // - Coverage enforcement via thresholds
 
  expect(typeof isCI).toBe("boolean");
  // => CI detection returns boolean
});

// vitest.config.ts - CI coverage enforcement
// test: {
//   coverage: {
//     provider: 'v8',
//     reporter: ['text', 'lcov'],
//     thresholds: {
//       lines: 80,
//       functions: 80,
//       branches: 80,
//       statements: 80,
//       // Coverage must meet these thresholds or Vitest exits with error code
//     },
//   },
// }
 
// CI command:
// npx vitest run --coverage
// => Runs tests, collects coverage, checks thresholds
// => Exits with error code 1 if thresholds not met
// => CI marks the job as failed
 
import { test, expect } from "vitest";
 
test("coverage threshold enforcement", () => {
  const thresholds = {
    lines: 80,
    functions: 80,
    branches: 80,
    statements: 80,
  };
  // => All dimensions must meet 80%
 
  const currentCoverage = {
    lines: 85,
    functions: 82,
    branches: 78,
    // => Below threshold!
    statements: 86,
  };
 
  const failures = Object.entries(thresholds).filter(
    ([key, threshold]) => currentCoverage[key as keyof typeof currentCoverage] < threshold,
  );
  // => Find dimensions below threshold
 
  expect(failures).toHaveLength(1);
  // => One dimension failed (branches at 78%)
  expect(failures[0][0]).toBe("branches");
  // => branches is the failing dimension
});
 
test("per-file coverage thresholds", () => {
  // Per-file thresholds catch concentrated gaps:
  // test: {
  //   coverage: {
  //     perFile: true,
  //     thresholds: { lines: 80 },
  //   },
  // }
  // => Each file must individually meet 80%
  // => Prevents one file with 0% hiding behind others
 
  const files = [
    { file: "auth.ts", coverage: 95 },
    { file: "utils.ts", coverage: 88 },
    { file: "legacy.ts", coverage: 45 },
    // => Below threshold!
  ];
 
  const belowThreshold = files.filter((f) => f.coverage < 80);
  expect(belowThreshold).toHaveLength(1);
  // => One file below threshold
  expect(belowThreshold[0].file).toBe("legacy.ts");
  // => legacy.ts needs more tests
});

// vitest.config.ts - Multi-reporter configuration
// test: {
//   reporters: process.env.CI
//     ? ['junit', 'json', 'github-actions']
//     : ['default'],
//   // => CI: machine-readable formats + GitHub annotations
//   // => Local: human-readable terminal output
//
//   outputFile: {
//     junit: './test-results/junit.xml',
//     json: './test-results/results.json',
//   },
// }
 
import { test, expect } from "vitest";
 
test("reporter selection by environment", () => {
  const isCI = process.env.CI === "true";
 
  const reporters = isCI ? ["junit", "json", "github-actions"] : ["default"];
  // => CI: structured output for tooling
  // => Local: colored terminal output
 
  if (isCI) {
    expect(reporters).toContain("junit");
    // => JUnit XML for CI test visualization
    expect(reporters).toContain("github-actions");
    // => GitHub Actions annotations on PR
  } else {
    expect(reporters).toContain("default");
    // => Terminal output for developers
  }
});
 
test("github-actions reporter annotations", () => {
  // github-actions reporter produces:
  // ::error file=src/math.test.ts,line=15::Test failed: expected 5, received 4
  // => GitHub displays this as inline PR annotation
  // => Developers see failures directly in the changed file
 
  const annotation = {
    level: "error",
    file: "src/math.test.ts",
    line: 15,
    message: "Test failed: expected 5, received 4",
  };
 
  expect(annotation.level).toBe("error");
  // => Error-level annotation
  expect(annotation.file).toContain(".test.ts");
  // => Points to specific test file
});

import { test, expect, vi } from "vitest";
 
test("CI auto-detection prevents watch mode", () => {
  // Vitest checks for CI environment variables:
  // - CI=true (GitHub Actions, GitLab CI, Travis CI)
  // - CONTINUOUS_INTEGRATION=true (Jenkins)
  // - TF_BUILD=true (Azure Pipelines)
 
  // In CI: npx vitest => runs once (no watch)
  // Locally: npx vitest => starts watch mode
 
  // Force single-run mode:
  // npx vitest run => always runs once
  // => Recommended for CI to be explicit
 
  const ciDetection = {
    envVars: ["CI", "CONTINUOUS_INTEGRATION", "TF_BUILD"],
    behavior: "auto-disable-watch",
    recommendation: "use 'vitest run' in CI scripts",
  };
 
  expect(ciDetection.envVars).toContain("CI");
  // => Standard CI environment variable
  expect(ciDetection.recommendation).toContain("vitest run");
  // => Best practice: explicit run command
});
 
test("process.exit behavior in CI", () => {
  // Vitest exit codes:
  // 0 = all tests passed
  // 1 = test failures or coverage below threshold
  // => CI uses exit code to determine pass/fail
 
  const exitCodes = {
    success: 0,
    testFailure: 1,
    coverageBelowThreshold: 1,
    configError: 1,
  };
 
  expect(exitCodes.success).toBe(0);
  // => Zero exit code = CI job passes
  expect(exitCodes.testFailure).toBe(1);
  // => Non-zero exit code = CI job fails
});

import { test, expect, vi } from "vitest";
 
// Simulating fs module mock
// vi.mock('fs/promises', () => ({
//   readFile: vi.fn(),
//   writeFile: vi.fn(),
//   mkdir: vi.fn(),
// }));
 
const mockFs = {
  readFile: vi.fn(),
  writeFile: vi.fn(),
  mkdir: vi.fn(),
};
 
// Function under test
async function loadConfig(fs: typeof mockFs, path: string): Promise<Record<string, unknown>> {
  try {
    const content = await fs.readFile(path);
    // => Reads config file
    return JSON.parse(content as string);
    // => Parses JSON content
  } catch {
    return { defaults: true };
    // => Returns defaults if file missing
  }
}
 
test("loads config from file", async () => {
  mockFs.readFile.mockResolvedValue('{"port": 3000, "host": "localhost"}');
  // => Mock file content
 
  const config = await loadConfig(mockFs, "/app/config.json");
  // => Calls mocked readFile
 
  expect(config).toEqual({ port: 3000, host: "localhost" });
  // => Passes: parsed file content
  expect(mockFs.readFile).toHaveBeenCalledWith("/app/config.json");
  // => Passes: correct path requested
});
 
test("returns defaults when file missing", async () => {
  mockFs.readFile.mockRejectedValue(new Error("ENOENT: no such file"));
  // => Simulate missing file
 
  const config = await loadConfig(mockFs, "/missing/config.json");
  // => readFile rejects, fallback triggers
 
  expect(config).toEqual({ defaults: true });
  // => Passes: default config returned
});
 
test("handles malformed JSON", async () => {
  mockFs.readFile.mockResolvedValue("not-valid-json");
  // => File exists but contains invalid JSON
 
  const config = await loadConfig(mockFs, "/app/config.json");
  // => JSON.parse throws, fallback triggers
 
  expect(config).toEqual({ defaults: true });
  // => Passes: graceful fallback on parse error
});

import { test, expect, vi, beforeEach, afterEach } from "vitest";
 
// Function that uses environment variables
function getApiUrl(): string {
  const env = process.env.NODE_ENV;
  // => Reads NODE_ENV
  const base = process.env.API_BASE_URL;
  // => Reads custom env var
 
  if (env === "production") return base || "https://api.production.com";
  // => Production URL
  if (env === "staging") return base || "https://api.staging.com";
  // => Staging URL
  return base || "http://localhost:3000";
  // => Default to local
}
 
beforeEach(() => {
  vi.stubEnv("NODE_ENV", "test");
  // => vi.stubEnv: safely mock environment variable
  // => Restores automatically in afterEach
});
 
afterEach(() => {
  vi.unstubAllEnvs();
  // => Restore all environment variables
});
 
test("returns production URL", () => {
  vi.stubEnv("NODE_ENV", "production");
  // => Override NODE_ENV for this test
 
  expect(getApiUrl()).toBe("https://api.production.com");
  // => Passes: production URL returned
});
 
test("returns staging URL", () => {
  vi.stubEnv("NODE_ENV", "staging");
 
  expect(getApiUrl()).toBe("https://api.staging.com");
  // => Passes: staging URL returned
});
 
test("returns custom base URL", () => {
  vi.stubEnv("NODE_ENV", "production");
  vi.stubEnv("API_BASE_URL", "https://custom.api.com");
  // => Override both variables
 
  expect(getApiUrl()).toBe("https://custom.api.com");
  // => Passes: custom URL takes precedence
});
 
test("defaults to localhost", () => {
  vi.stubEnv("NODE_ENV", "development");
 
  expect(getApiUrl()).toBe("http://localhost:3000");
  // => Passes: default local URL
});

import { test, expect, vi, beforeEach, afterEach } from "vitest";
 
beforeEach(() => {
  vi.useFakeTimers();
  // => Enables fake timers (includes Date)
});
 
afterEach(() => {
  vi.useRealTimers();
  // => Restore real timers
});
 
test("mock Date.now()", () => {
  vi.setSystemTime(new Date("2026-04-05T10:00:00Z"));
  // => Set fake system time
 
  expect(Date.now()).toBe(new Date("2026-04-05T10:00:00Z").getTime());
  // => Passes: Date.now() returns fake time
 
  const now = new Date();
  // => Creates Date with fake time
  expect(now.toISOString()).toBe("2026-04-05T10:00:00.000Z");
  // => Passes: new Date() uses fake time
});
 
test("mock Date for time-dependent logic", () => {
  vi.setSystemTime(new Date("2026-12-25T00:00:00Z"));
  // => Set to Christmas
 
  function isHoliday(): boolean {
    const now = new Date();
    return now.getMonth() === 11 && now.getDate() === 25;
    // => Check if December 25th
  }
 
  expect(isHoliday()).toBe(true);
  // => Passes: fake date is Christmas
 
  vi.setSystemTime(new Date("2026-07-04T00:00:00Z"));
  // => Advance to July 4th
  expect(isHoliday()).toBe(false);
  // => Passes: not Christmas anymore
});
 
test("mock Math.random for deterministic tests", () => {
  // Mock Math.random with predictable sequence
  const mockRandom = vi.spyOn(Math, "random");
  mockRandom.mockReturnValueOnce(0.1).mockReturnValueOnce(0.5).mockReturnValueOnce(0.9);
  // => Sequence: 0.1, 0.5, 0.9
 
  function generateId(): string {
    return Math.random().toString(36).slice(2, 8);
    // => Random ID from Math.random
  }
 
  const id1 = generateId();
  const id2 = generateId();
  const id3 = generateId();
 
  // IDs are deterministic with mocked random
  expect(id1).toBe((0.1).toString(36).slice(2, 8));
  // => Passes: predictable from mocked 0.1
  expect(id2).toBe((0.5).toString(36).slice(2, 8));
  // => Passes: predictable from mocked 0.5
 
  mockRandom.mockRestore();
  // => Restore real Math.random
});

import { test, expect, vi } from "vitest";
 
// MSW setup (conceptual - real MSW requires package install):
// import { setupServer } from 'msw/node';
// import { http, HttpResponse } from 'msw';
//
// const server = setupServer(
//   http.get('/api/users', () => {
//     return HttpResponse.json([
//       { id: 1, name: 'Alice' },
//       { id: 2, name: 'Bob' },
//     ]);
//   }),
// );
//
// beforeAll(() => server.listen());
// afterEach(() => server.resetHandlers());
// afterAll(() => server.close());
 
// Self-contained MSW pattern demonstration
type Handler = {
  method: string;
  path: string;
  response: unknown;
};
 
function createMockServer(handlers: Handler[]) {
  // => Simulates MSW server concept
  return {
    handlers,
    handleRequest: (method: string, path: string) => {
      const handler = handlers.find((h) => h.method === method && h.path === path);
      // => Find matching handler
      if (handler) return { status: 200, data: handler.response };
      return { status: 404, data: null };
      // => 404 for unmatched routes
    },
    resetHandlers: () => {
      // => Reset to initial handlers
    },
  };
}
 
test("MSW intercepts API calls", () => {
  const server = createMockServer([
    { method: "GET", path: "/api/users", response: [{ id: 1, name: "Alice" }] },
    { method: "POST", path: "/api/users", response: { id: 2, name: "Bob" } },
  ]);
  // => Create server with handlers
 
  const getResult = server.handleRequest("GET", "/api/users");
  expect(getResult.status).toBe(200);
  // => Passes: GET handler matched
  expect(getResult.data).toEqual([{ id: 1, name: "Alice" }]);
  // => Passes: returns mock user data
 
  const postResult = server.handleRequest("POST", "/api/users");
  expect(postResult.status).toBe(200);
  // => Passes: POST handler matched
 
  const notFound = server.handleRequest("GET", "/api/unknown");
  expect(notFound.status).toBe(404);
  // => Passes: unhandled route returns 404
});

import { test, expect } from "vitest";
 
// Monorepo structure:
// packages/
//   ui/          => Component library (happy-dom)
//   api/         => API client (node)
//   shared/      => Shared utilities (node)
//   e2e/         => E2E tests (playwright)
 
// vitest.workspace.ts:
// export default [
//   {
//     extends: './vitest.config.ts',
//     test: {
//       name: 'ui',
//       root: './packages/ui',
//       environment: 'happy-dom',
//     },
//   },
//   {
//     extends: './vitest.config.ts',
//     test: {
//       name: 'api',
//       root: './packages/api',
//       environment: 'node',
//     },
//   },
//   {
//     extends: './vitest.config.ts',
//     test: {
//       name: 'shared',
//       root: './packages/shared',
//       environment: 'node',
//     },
//   },
// ];
 
test("monorepo workspace configuration", () => {
  const workspace = {
    projects: [
      { name: "ui", environment: "happy-dom", root: "packages/ui" },
      { name: "api", environment: "node", root: "packages/api" },
      { name: "shared", environment: "node", root: "packages/shared" },
    ],
  };
 
  expect(workspace.projects).toHaveLength(3);
  // => Three packages configured
  expect(workspace.projects[0].environment).toBe("happy-dom");
  // => UI package uses DOM environment
  expect(workspace.projects[1].environment).toBe("node");
  // => API package uses node environment
});
 
test("workspace commands", () => {
  // npx vitest run                  => Run all projects
  // npx vitest run --project=ui     => Run only UI tests
  // npx vitest run --project=api    => Run only API tests
  // npx vitest run --changed        => Run affected projects
 
  const commands = {
    all: "npx vitest run",
    single: "npx vitest run --project=ui",
    affected: "npx vitest run --changed",
  };
 
  expect(commands.all).not.toContain("--project");
  // => All projects: no filter
  expect(commands.single).toContain("--project=ui");
  // => Single project: filter flag
  expect(commands.affected).toContain("--changed");
  // => Affected: git-based detection
});

// vitest.config.ts - Dependency optimization
// test: {
//   deps: {
//     optimizer: {
//       web: {
//         include: ['@testing-library/react'],
//         // => Pre-bundle for browser environment
//       },
//       ssr: {
//         include: ['lodash-es'],
//         // => Pre-bundle for SSR/node environment
//       },
//     },
//     external: ['fsevents'],
//     // => Don't process these modules (native addons)
//     inline: ['my-custom-package'],
//     // => Bundle these into test files (ESM compatibility)
//   },
// }
 
import { test, expect } from "vitest";
 
test("dependency optimization concepts", () => {
  const depConfig = {
    optimizer: {
      // Pre-bundling reduces startup time
      web: ["@testing-library/react", "react-dom"],
      // => Browser-targeted packages
      ssr: ["lodash-es", "date-fns"],
      // => Node-targeted packages
    },
    external: ["fsevents", "esbuild"],
    // => Skip processing for native modules
    inline: ["my-esm-only-package"],
    // => Inline CJS-incompatible packages
  };
 
  expect(depConfig.optimizer.web).toContain("@testing-library/react");
  // => Testing Library pre-bundled for speed
  expect(depConfig.external).toContain("fsevents");
  // => Native modules externalized
});
 
test("resolving ESM/CJS issues", () => {
  // Common issue: package uses ESM but test env expects CJS
  // Solution 1: deps.inline (force bundle)
  // Solution 2: deps.optimizer.ssr.include (pre-bundle)
  // Solution 3: alias in vite config
 
  const solutions = {
    inline: "Force-bundle the package into test",
    optimize: "Pre-bundle for compatibility",
    alias: "Map import to specific entry point",
  };
 
  expect(Object.keys(solutions)).toHaveLength(3);
  // => Three approaches to ESM/CJS conflicts
});

import { test, expect } from "vitest";
 
test("module graph concepts", () => {
  // Vitest uses Vite's module graph to:
  // 1. Determine which tests to re-run on file change
  // 2. Pre-bundle dependencies for faster loading
  // 3. Invalidate cached transforms when dependencies change
 
  const moduleGraph = {
    "math.test.ts": {
      imports: ["math.ts", "vitest"],
      // => Test imports math module and vitest
      importedBy: [],
      // => No one imports the test file
    },
    "math.ts": {
      imports: ["constants.ts"],
      // => math.ts imports constants
      importedBy: ["math.test.ts", "calculator.ts"],
      // => Used by test and other modules
    },
    "constants.ts": {
      imports: [],
      // => Leaf module (no imports)
      importedBy: ["math.ts"],
      // => Used by math module
    },
  };
 
  // When constants.ts changes:
  // 1. math.ts is invalidated (imports constants.ts)
  // 2. math.test.ts is invalidated (imports math.ts)
  // 3. math.test.ts re-runs
 
  const affectedByConstantsChange = ["math.ts", "math.test.ts"];
  expect(affectedByConstantsChange).toContain("math.test.ts");
  // => Test re-runs when transitive dependency changes
});
 
test("HMR-powered test re-execution", () => {
  // Watch mode uses Vite HMR for fast re-execution:
  // 1. File saved
  // 2. Vite HMR detects change
  // 3. Module graph identifies affected tests
  // 4. Only affected test modules are re-loaded
  // 5. Tests re-run with new code
 
  const hmrFlow = ["file-saved", "hmr-detect", "graph-walk", "module-invalidate", "test-re-run"];
 
  expect(hmrFlow).toHaveLength(5);
  // => Five-step HMR flow
  expect(hmrFlow[0]).toBe("file-saved");
  // => Triggered by file save
  expect(hmrFlow[4]).toBe("test-re-run");
  // => Ends with test re-execution
});

import { test, expect, vi } from "vitest";
 
// Vite plugin interface (simplified)
interface VitePlugin {
  name: string;
  transform?: (code: string, id: string) => string | null;
  resolveId?: (source: string) => string | null;
}
 
// Example plugin: auto-import CSS modules
function cssAutoImport(): VitePlugin {
  return {
    name: "css-auto-import",
    // => Plugin name for debugging
    transform(code: string, id: string): string | null {
      if (!id.endsWith(".tsx")) return null;
      // => Only process .tsx files
      // => Return null to skip transformation
 
      if (code.includes("className=")) {
        // => File uses CSS classes
        const cssFile = id.replace(".tsx", ".module.css");
        return `import styles from '${cssFile}';\n${code}`;
        // => Auto-import corresponding CSS module
      }
      return null;
      // => No transformation needed
    },
  };
}
 
test("plugin transforms TSX files", () => {
  const plugin = cssAutoImport();
  // => Create plugin instance
 
  const code = 'const App = () => <div className="container">Hello</div>';
  const result = plugin.transform!(code, "App.tsx");
  // => Transform code as if processing App.tsx
 
  expect(result).toContain("import styles from");
  // => Passes: CSS import was added
  expect(result).toContain("App.module.css");
  // => Passes: correct CSS file path
});
 
test("plugin skips non-TSX files", () => {
  const plugin = cssAutoImport();
 
  const result = plugin.transform!("const x = 1;", "util.ts");
  // => Process a .ts file (not .tsx)
 
  expect(result).toBeNull();
  // => Passes: plugin skips non-TSX files
});
 
test("plugin skips files without className", () => {
  const plugin = cssAutoImport();
 
  const code = "const App = () => <div>No classes</div>";
  const result = plugin.transform!(code, "Simple.tsx");
  // => TSX file but no className usage
 
  expect(result).toBeNull();
  // => Passes: no transformation when className absent
});

import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
 
// Comprehensive cleanup pattern
beforeEach(() => {
  vi.clearAllMocks();
  // => Clears call history on all mocks
  // => Preserves mock implementations
});
 
afterEach(() => {
  vi.restoreAllMocks();
  // => Restores original implementations
  // => Removes all mock overrides
 
  vi.unstubAllEnvs();
  // => Restores environment variables
 
  vi.useRealTimers();
  // => Restores real Date, setTimeout, etc.
});
 
// Singleton isolation pattern
describe("singleton isolation", () => {
  // Problem: module-level singletons persist between tests
  // let instance: MyService | null = null;
  // export function getInstance() {
  //   if (!instance) instance = new MyService();
  //   return instance;
  // }
 
  // Solution: vi.resetModules() before re-importing
  it("gets fresh singleton with resetModules", async () => {
    vi.resetModules();
    // => Clears module cache
    // => Next import creates new module instance
 
    // const { getInstance } = await import('./singleton');
    // => Fresh import, new singleton created
 
    const singletonConcept = {
      beforeReset: "shared-instance",
      afterReset: "fresh-instance",
    };
 
    expect(singletonConcept.afterReset).toBe("fresh-instance");
    // => Passes: resetModules creates fresh state
  });
});
 
// Global state cleanup
describe("global state cleanup", () => {
  it("test 1 modifies globals", () => {
    (globalThis as Record<string, unknown>).__testFlag = true;
    // => Sets global flag (leaks to other tests!)
    expect((globalThis as Record<string, unknown>).__testFlag).toBe(true);
    // => Passes in this test
  });
 
  afterEach(() => {
    delete (globalThis as Record<string, unknown>).__testFlag;
    // => Clean up global modifications
  });
 
  it("test 2 should not see test 1 globals", () => {
    expect((globalThis as Record<string, unknown>).__testFlag).toBeUndefined();
    // => Passes: global cleaned up by afterEach
  });
});
 
// Comprehensive isolation checklist
describe("isolation verification", () => {
  it("verifies no mock leaks", () => {
    // After vi.restoreAllMocks():
    expect(vi.isMockFunction(console.log)).toBe(false);
    // => Passes: console.log is not mocked
  });
 
  it("verifies no timer leaks", () => {
    // After vi.useRealTimers():
    const before = Date.now();
    // => Real Date.now() (not mocked)
    expect(before).toBeGreaterThan(0);
    // => Passes: real time returned
  });
});